Natural fibres for composites

Materials World magazine

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1 Nov 2008

A UK consortium has announced the development of its first generation of high performance composites made from natural renewable fibres such as hemp and flax. This could help create more environmentally friendly structural components in the transport, medical and construction sectors, say researchers.

The COMBINE project is halfway through its 2.5-year run and is said to have overcome the difficulties in producing natural fibre and thermoplastic-based composites with the required mechanical properties.

Lead partner NetComposites Ltd, based in Chesterfield, UK, claims that the consortium’s biocomposite made from 30vol%/30wt% unidirectional flax to polylactic acid (PLA) can compete with conventional glass-reinforced plastic at 18vol%/30wt%. The former has a tensile modulus of 18GPa and strength of 180MPa, while the latter has a tensile modulus of 7GPa and strength of 120MPa.

Dr Brendon Weager, Technical Manager at NetComposites, says, ‘Natural fibres are characteristically discontinuous and tend to be highly twisted in order to obtain sufficient strength from the relatively short fibres – sometimes as little as a few millimetres. This is acceptable for clothing and building insulation but not for composite reinforcement, where you get improved mechanical strength and stiffness when the fibres are aligned in the direction of the load’.

The twisted nature of the yarns also poses problems in impregnating the fibres with viscous thermoplastics such as polypropylene (PP) and PLA.

The team has developed novel spinning techniques to convert hemp and flax fibres into continuous straight aligned yarns with low twist, although Weager was unable to reveal more about the technique at this stage.

He adds, ‘One challenge is to improve the compatibility of the fibres with the thermoplastics by treating them with various cleaning agents, bleaching, and caustic or plasma treatment’. The brittle nature of PLA also requires an impact modifier.

The result is two woven flax-fibre-reinforced composites based on PLA and PP.

The materials are not commercially available yet, and further work will include process optimisation for economical mass production, refinement of the spinning and weaving techniques for optimum properties, and exploration of painting, bonding and moulding techniques. Investigating the recyclability and biodegradability of these composites is also on the agenda.

Partners Fairline Boats Ltd and Lightweight Medical based in Oundle and Lanarkshire, respectively, have begun to develop demonstrator parts using the materials. There is also an opportunity for a moulding company to aid this process.

Environmental credentials?

The UK’s COMBINE project aims to produce environmentally friendly composites that can compete economically with carbon and glass-fibre reinforced materials. Brendon Weager of NetComposites Ltd explains that natural fibres consume less energy in fibre production than glass, which needs to be fired. The growth, harvesting, retting and combing of natural fibres is conducted at room temperature. ‘While there are some impacts that arise from the cultivation of the hemp, the energy demand is low,’ says Weager.

Moreover, in the next phase of the project, researchers at Queen Mary, University of London, will explore the biodegradability and recyclability of the new composites. Ton Peijs, Professor of Materials at the University, acknowledges that this is where the environmental credentials of the materials become more complex. ‘If we use biodegradable plastics (PLA) and fibres, and there is not a composting infrastructure to support it, they end up in landfill, rot and create methane.’

He suggests incineration for energy is another option, but recycling should be at the top of the waste hierarchy. He says, ‘You cannot separate out the natural fibres and thermoplastics. So we are going to look at how you can cut the composite up, grind it and then injection mould it into another component’.